Direct Digital Synthesizers (DDS) are used to create sine waves and derivative waves that are needed for Automated-Test-Equipment (ATE), software-defined radio, medical imaging, radar systems, and other applications. While sine waves are a part of the natural world and thus are common in analog systems, digital systems rely on 1's and 0' and do not have fundamental physical properties that cause sine waves to be generated. Thus generating an approximation of a sine wave is more problematic in digital systems.
FIG. 1 shows a prior art Direct Digital Synthesizer (DDS) system. A frequency control word K may represent a phase and is accumulated by adder 12 in phase accumulator 10. The accumulated phase is input to converter 14, which converts the accumulated phase to a sine wave. Since DDS core is a digital system, the accumulated phase and sine wave are digital values, not continuous voltages. Digital-to-Analog Converter (DAC) 16 converts the digital value that represents the sine wave to and analog voltage or current. Since DDS core 18 has discrete steps in the digital values, the final analog waveform shows steps in voltage, although these can be reduced by an analog filter such as a capacitor on the output of DAC 16, or by reducing the step size of the digital values in DDS core 18, such as by increasing the number of digital bits.
Converter 14 may be a digital circuit that uses an approximation of the sine wave function. For example, a read-only memory (ROM) may store amplitude values of the sine wave function for a range of phase input values. However, the size of the ROM may be excessive, so more complex functions may be used to approximate the amplitude of the sine wave using fewer entries in the ROM. The digital phase input may be broken into several component parts and partial sine and cosine functions of the component parts are generated and multiplied and added together. Even more complex functions such as Taylor series have been used. Additional adders, subtractors, multipliers, and their control logic can make these implementations expensive and slow the system.
The compression ratio is the ratio of the size of an ideal ROM of the DDS to the size of entries in all ROMs that this architecture uses, for the same phase and amplitude resolution.
The difference in signal strength between a fundamental and a spur in a spectrum graph of the sine wave generator is the Spur-Free Dynamic Range (SFDR). A larger SFDR is important for better high-speed performance of the DAC and other components.
Larger spurs often occur at harmonics of the fundamental frequency, especially the second and third order harmonics. It is desirable to increase SFDR.
What is desired is a DDS sine wave generator that does not require a huge ROM and many digital bits in the phase input. A digital sine wave generator with a good compression ratio and SFDR is desired. A digital synthesizer with a sine wave generator that is not too complex to operate at a high speed is desirable.